This invention relates to impedance measurement arrangements and, more particularly, to a method and apparatus for measuring capacitance.
Numerous techniques are known in the art for obtaining capacitance measurements. For example, bridge circuits which include either manual or automatic balancing arrangements have been employed for this purpose. Bridge circuits which employ manual balancing are undesirable for use in automatic systems. Bridge circuits which employ automatic balancing are typically complex and expensive. Therefore, they are unattractive for use in automated systems from an economics standpoint.
More recently, capacitance measurements have been made by employing a periodic ramp signal having a known slope which is supplied to a network including an unknown capacitance. Typically, the capacitance to be measured is connected in a circuit to form a differentiator. Then, the output voltage from the differentiator is detected and measured. For a "pure" capacitance, the differentiator output is a square wave having an amplitude proportional to the unknown capacitance value. This prior ramp signal technique yields satisfactory results for measurements of pure capacitance values. However, unsatisfactory results are obtained when measuring unknown capacitance values of networks including other impedance components, for example, series resistance, series inductance and shunt resistance. This is because the current developed in such networks in response to the ramp signal does not have a constant amplitude. Thus, the prior known ramp signal measurement technique is unacceptable for obtaining representative capacitance measurements of telephone subscriber loops and the like which include impedance components in addition to capacitance.